D-Lysergic acid diethylamide (LSD) and its psychotropically weak 2-bromo derivative (BOL) were compared in terms of their relative potencies in inhibiting serotonergic (5 - hydroxytryptamine) neurones verses neurones in two representative areas receiving an indentified serotonergic input-- the amygdala and the ventral lateral geniculate. Drug ejection and unit recording were accomplished by means of 5-barrelled micropipettes. For each neurone tested, the ejection currents of LSD and BOL were within about 20 sec. Serotonin had a high inhibitory potency in all the areas studied. At equal ejection currents, serotonin and LSD (the latter ionically diluted 100 fold with NaCl) were equipotent in inhibiting serotonergic neurones in the dorsal rahpe nucleus; BOL had substantially less inhibitory activity at equal or even higher ejection currents. In the amygdala and ventral lateral geniculate both LSD and BOL were considerably less potent than serotonin in inhibition neuronal firing. Moreover, neither BOL nor LSD produced any marked blockade of serotonin in these areas It is concluded that differences in direct actions upon sertonergic (raphe) neurones discriminates best between LSD and BOL. D - Lysergic acid diethylamide but not BOL had a highly potent serotonin agonist-like inhibitory aciton on serotonergic neurones, while both drugs had relatively weak inhibitory actions (in comparison with serotonin) on postsynaptic cells in the ventral lateral geniculate and the amygdala. The greater potency of LSD in inhibiting serotonergic neruones parallels the greater hallucinogenic potency of LSD compared with BOL.